The present invention relates, in general, to an injection molding device for processing plastic material.
An injection molding device of a type involved here is disclosed in German Pat. No. DE-PS 1 142 229 and includes a plasticizing unit having a continuously operating extruder and an injection unit having a discontinuously operating piston and cylinder arrangement. As no plastic material can be supplied during injection process by the injection unit, a pressurized storage space is placed between the extruder and the injection unit for accumulating the plasticized plastic material that continuously builds up.
FIG. 1 shows an injection molding device of this conventional type with an injection unit, which has an injection cylinder 65 and an injection piston 60 reciprocating in the injection cylinder 65. The interior of the injection cylinder 65 is connected via channels 90 and a check valve 35 with a buffer reservoir, generally designated by reference numeral 25. The buffer reservoir 25 includes a tubular barrel 30 and a screw 70, which is rotatably received in the barrel 30 and movable along the axis of the barrel 30. At its end distal to the injection unit, the barrel 30 of the buffer reservoir is in communication with a cylindrical chamber 20 under pressure. A channel 10 connects the interior of the barrel 30 with a continuously operating extruder (not shown).
In operation, plasticized plastic material is fed via the channel 10 into the screw flight of the screw 70. As the screw 70 rotates, plastic material is advanced in the direction of the check valve 35 and flows via the channels 90 into the interior of the injection cylinder 65 while the piston 60 is displaced. During the injection process, the piston 60 is operated to inject plastic material via an injection head 50 into a molding tool (not shown), which is attached to a platen 80. The check valve 35 prohibits hereby a passage of plastic material from the screw 70 into the channels 90. As the screw 70 continues to rotate, a buffer 40 is formed ahead of the check valve 35 and forces the screw 70 to move in the direction of the biased cylindrical chamber 20.
As soon as the injection process is over, the check valve 35 opens and the biased cylindrical chamber 20 moves the screw 70 in the direction of the check valve 35 so that the injection cylinder 65 can be charged again.
This prior art suffer shortcomings because even though the extruder operates continuously, irregularities are encountered during the transport of plastic material so that sensitive plastic materials may be damaged.
German Pat. No. DE-PS 1 142 229 also refers to the option to use a continuously operating extruder for alternating supply of plastic material to two injection units. When reversing a valve, which is positioned between the extruder and both injection units, momentary pressure surges may be encountered that also adversely affect the plastic material.
German Pat. No. 197 15 229 discloses a method and apparatus for linking a continuous plasticizing operation with a cyclical, discontinuous melt discharge, by using an interposed buffer reservoir for receiving and storing continuously plasticized and conveyed plastic material.
It would be desirable and advantageous to provide an improved injection molding device, which obviates prior art shortcomings and which is constructed to substantially eliminate the occurrence of pressure surges and inhomogeneities in the plastic material